Petroleum refiners often produce desirable products, such as turbine fuel, diesel fuel and other products known as middle distillates, as well as lower boiling hydrocarbonaceous liquids, such as naphtha and gasoline, by hydrocracking a hydrocarbon feedstock derived from crude oil or heavy fractions thereof. Feedstocks most often subjected to hydrocracking are the gas oils and heavy gas oils recovered from crude oil by distillation. A typical heavy gas oil comprises a substantial portion of hydrocarbon components boiling above 370° C. (700° F.). A typical vacuum gas oil has a boiling point range between 315° C. (600° F.) and about 565° C. (1050° F.).
Hydrocracking is generally accomplished by contacting in a hydrocracking reaction vessel or zone the gas oil or other feedstock to be treated with a suitable hydrocracking catalyst under conditions of elevated temperature and pressure in the presence of hydrogen so as to yield a product containing a distribution of hydrocarbon products desired by the refiner. Generally, middle distillates are the most desirable products, naphtha and gasoline are less desirable, and light ends comprising hydrocarbons with 1 to 4 carbons are the least desirable. The operating conditions and the hydrocracking catalyst within the hydrocracking reactor influence the yield of the hydrocracked products.
Traditionally, the fresh feedstock for a hydrocracking process is first introduced into a denitrification and desulfurization reaction zone particularly suited for the removal of sulfur and nitrogen contaminants. Subsequently, the feedstock is introduced into a hydrocracking zone containing hydrocracking catalyst. Within the hydrocracking zone, heavier components of the hydrocarbon feedstock tend to undergo cracking before lighter components of the hydrocarbon feedstock. Specifically, cracking catalysts show at least some selectivity toward cracking heavier components, i.e., components having higher boiling temperatures, over lighter components, i.e., components having lower boiling temperatures. Thermodynamically, cracking of heavier hydrocarbon components is preferred as such cracking results in a larger increase in entropy. Conventional processes tend to have poor selectivity toward middle distillates.
Accordingly, it is desirable to provide methods and apparatuses for upgrading hydrocarbon streams with improved efficiency. In addition, it is desirable to provide methods and apparatuses that economically hydrocrack hydrocarbon streams. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.